1. Field of the Invention
This invention relates to a doctoring apparatus of the type employed in the processing of paper, textiles and other like industrial products.
2. Description of the Prior Art
In the conventional doctoring apparatus, the working edge of a doctor blade is applied to a rotating surface from which materials are to be removed. In some cases, the doctored material may consist of a sheet or web being processed on the rotating surface, whereas in other cases the doctored material may consist of contaminants accumulating on the surface.
Among the critical factors contributing to an effective doctoring operation are optimum blade angle, and a uniform blade loading pressure along the entire length of the blade/surface contact line. If the blade angle is too large, the blade will have a tendency to dig or jam into the doctored surface and thus cause serious damage, whereas if the blade angle is too small, the material to be doctored will escape beneath the blade, causing machine damage and/or loss of production. Excessive blade loading pressure will accelerate wear of both the blade and the doctored surface, in addition to increasing the power required to overcome the accompanying increased friction. Insufficient or non-uniform blade loading pressure again may allow material to escape beneath the blade.
A number of attempts have been made at providing a doctoring apparatus capable of maintaining an optimum blade angle and uniform blade loading pressure. One such apparatus is illustrated in FIG. 1, where a doctor back 2 has a generally L-shaped configuration with end shafts 4 supported in bearings 6 for rotation about a first axis A.sub.1. The bearings 6 are carried on a support structure 10 and are adjustable to accommodate shifting of axis A.sub.1 in the direction indicate by arrow 12. The doctor back is rotated about axis A.sub.1 by any conventional means, for example pneumatically actuated piston-cylinder units 14.
The doctor back carries a blade holder 16. As can be better seen in FIG. 2, the blade holder includes a relatively rigid and inflexible top plate 18 and an underlying rigid and inflexible jaw 20. A doctor blade 22 is removably received and supported between the top plate 18 and jaw 20. The top plate 18 is secured to the doctor back by hold down screws 24.
During initial set up of the machine, the piston-cylinder units 14 are pressurized to rotate the doctor back in a clockwise direction as viewed in FIG. 1, thereby loading the doctor blade 22 against the surface S being doctored. The surface S may have localized low spots. Thus, adjustable set screws 26 are provided to "fit" the blade to these low spots.
With this type of doctoring apparatus, blade loading pressure is a function of the force being exerted by the piston-cylinder units 14, and the blade angle .alpha. is a function of the position of the axis A.sub.1 relative to the surface S. In order to adjust the blade angle, the bearings 6 must be shifted in the direction indicated at 12. Thus, once the machine is in operation, if it is determined that the blade angle needs further adjustment, the entire machine must be shut down to provide maintenance personnel with access to the bearings 6, thereby resulting in protracted and costly lost production time. A further drawback with this type of apparatus is that because the doctor blade 22 is held between the relatively rigid and inflexible top plate 18 and jaw 20, it lacks the flexibility to satisfactorily accommodate irregularities in the surface S being doctored, this despite the ability to perform localized adjustments by means of the set screws 26.
A modified prior art blade holder is disclosed in FIG. 3. Here, the rear edge of the doctor blade 22 is supported on a liquid-filled and completely sealed flexible reaction tube 28. Although this enhances the ability of the blade to conform to localized irregularities of the surface S being doctored, the same problems remain with respect to the difficulty of changing the blade angle .alpha..
Another prior art doctoring apparatus is illustrated in FIG. 4A. This apparatus is similar to that shown in FIG. 1 in that it too has a doctor back 2 with end shafts 4 mounted in bearings 6 for rotation about an axis A.sub.1 under the influence of piston-cylinder units 14. Here, however, the piston-cylinder units 14 do not load the doctor blade 22 against the surface S. Instead, the piston-cylinder units merely serve to locate the working edge of the doctor blade close to but spaced from surface S by pulling the doctor back against an adjustable stop 29. Although not illustrated, it will be understood that the same result could be achieved by causing the piston cylinder units to "bottom out", or by substituting turnbuckles for the piston cylinder units.
The apparatus of FIG. 4A includes a further modified blade holder of the type shown in FIG. 4B and 4C. Here, the blade holder includes a series of pressure fingers 30 spaced along a common axis A.sub.2. The doctor blade 22 is held between the fingers 30 and a flexible top plate 32, and pneumatically inflated tubes 34,36 extend along opposite sides of the axis A.sub.2 between the fingers 30 and the doctor back 10.
With this type of doctoring apparatus, once the piston-cylinder units 14 have been actuated to fix the doctor back 2 at a selected position, e.g., against stop 29, final blade loading against surface S is achieved by pneumatically inflating tube 34 (with an accompanying deflation of tube 36). The level of blade loading is thus a function of the air pressure in tube 34 acting across a contact width W.
However, as illustrated in FIG. 4C, if the doctor back 2 remains fixed while the blade angle .alpha. increases, either because of blade wear or because process requirements mandate such an increase, the tube 34 will necessarily expand to accommmodate rotation of the fingers 30 about axis A.sub.2. This in turn will cause the contact width W to decrease with an accompanying decrease in the level of blade loading. In other words, with this apparatus, the level of blade loading is inversely proportional to blade angle. If the level of blade loading becomes inadequate, there is a danger that the blade will lift off of the surface S, with potentially disastrous consequences.
In order to keep the contact width W within an acceptable range, the doctor back 2 must be readjusted by shifting the bearings 6 and adjusting the stop 29, thereby again necessitating protracted down time and lost production.
The prior art apparatus of FIGS. 4A-4C suffers from still other drawbacks. For example, the combination of individual pressure fingers 30 acted upon by a pneumatically inflated tube 34 results in maximum blade flexibility. However, a sudden impact at any localized zone along the blade length can cause the affected blade section to be lifted off of the surface S, again with potentially disastrous consequences.
Also, the axis A.sub.2 is conventionally defined by a long rod or shaft threaded through the individual fingers from one side of the machine to the other. This is a difficult and laborous procedure which contributes significantly to machine down time when maintenance, e.g., clearing of the holder is required.
U.S. Pat. Nos. 3,163,878; 3,748,686; and 3,803,665 illustrate other examples of doctoring apparatus employing pneumatically inflated tubes to load the doctor blades against the surfaces being doctored.